1. Field of the Disclosure
The present disclosure relates to a wideband beam forming device and method and to a wideband beam steering device and method. The present disclosure relates further to an active imaging device and a corresponding method for imaging a scene. Still further, the present disclosure relates to a computer readable non-transitory medium storing such a computer program.
2. Description of Related Art
Active imaging systems including beam forming are becoming more and more popular at ultrasonic, microwave, millimeter and terahertz frequencies for a number of applications including medical and security applications.
The arrangement of transmitter (herein also called “transmit element”) and receiver (herein also called “receive element”) in an active imaging system may take on many different forms. In an embodiment relevant for the present disclosure multiple transmitters and receivers work together to form a MIMO radar (or MIMO active imaging system). There are predominately two different types of MIMO radars. The first type is called statistical MIMO, in which the antennas (generally the “transmit elements” and the “receive elements”) are placed far apart from each other to provide different views of the object (generally the “scene”). The second type of MIMO is called beam forming (or co-located) MIMO in which the antennas are placed close to each and act together to form a “virtual” beam forming array.
For beam forming to be performed at the receiver in a beam forming device, typically multiple receiving antennas are used to perform beam forming. Such an arrangement may use one transmitting antenna or may use multiple transmitting antennas in combination with the multiple receiving antennas to perform MIMO beam forming. Alternatively, beam forming maybe be performed exclusively at the transmitter in a beam steering device with the use of multiple transmitting antennas. It shall be noted here that hereinafter reference is made to a beam steering device and method when beam forming (beam steering) is solely performed on the transmitter side, and that hereinafter reference is made to a beam forming device and method when beam forming is solely performed on the receiver side. In more detail, beam forming and beam steering both use weights to form a beam pattern. The difference is, on the transmitter side, beam steering generally uses only one weight vector to form one beam pattern and radiates it to the space (i.e. only one radiation pattern can exist physically at one time). On the receiver side, digital beam forming could use multiple weight vectors to form multiple beam patterns at the same time in parallel and get multiple beam formed outputs.
Beam forming in one dimension is typically combined with other techniques (i.e. synthetic aperture radar) to form a 2D image. Alternatively, beam forming can be performed in two dimensions to form a 2D image. To yield a full 3D image of an object (or a 2D image with additional distance/depth information), such arrangements typically transmit a wideband continuous waveform (i.e. frequency modulated continuous wave (FMCW)) or a wideband pulse to provide ranging (distance) information.
Most conventional methods to achieve wideband beam forming utilise filter banks. These methods work either in the time domain with two-dimensional filter banks or operate in the frequency domain (after FFT), filtering each band separately. They strive to either split the wideband signal into many sub-bands which are individually narrowband filtered or they strive to provide frequency invariant beam forming.
Exemplary methods to achieve wideband beam forming for radar and communications systems are described in the following documents.
Z. Hu et al, “Design of Look-up Table Based Architecture for Wideband Beam forming”, 2010 International Waveform Diversity and Design Conference Aug. 8-13, 2010 describes how to achieve wideband beam forming by either using time filtering in 2D or by using a FFT followed by 1D filtering. In particular, it describes using a 2D filter bank after the ADC. To perform wideband beam forming in this way, the sampling rate of the ADC needs to be more than two times the bandwidth of the transmitted signal.
Z. Hu et al, “Robust Wideband Beam forming”, IEEE National Aerospace and Electronics Conference, Jul. 14-16, 2010 describes a method to achieve robust optimisation of wideband beam forming. In the same way as the above mentioned document of Z. Hu wideband beam forming is achieved using a 2D filter after the ADC.
B. Matthews et al, “Wideband Radar Adaptive Beam forming using Frequency Domain derivative based updating”, Proceedings of Software defined Radio Forum, November 2007 describes a method to perform wideband adaptive beam forming to mitigate the effect of wideband jammers. In particular, the proposed methods include those described in the above mentioned documents of Z. Hu (i.e. 2D filtering or frequency domain filtering) and additionally propose the computation of a wideband co-variance matrix to find the optimum weights.
M.-S. Lee, “Wideband Capon Beam forming for a planar phased radar array with antenna switching”, ETRI Journal, Volume 31, Number 3, June 2009 describes a FMCW system which uses a 2D antenna array in which each row of the 2D antenna array is switched successively to a beam forming array. The weights for the beam former are changed depending upon which row is selected and the correspondingly frequency difference in the chirp waveform which may have occurred during this time. Additionally the paper only addresses the use of capon beam forming which requires calculating the covariance of the received signal.
U.S. Pat. No. 6,940,917 B2 describes a wideband beam steering multi-carrier (or OFDM) communication system. Each of the different frequency carriers are transmitted at the same time and are assigned a different weighting vector.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.